Tehran, which could now purchase weapons from Russia, China and elsewhere, has hailed the development as a diplomatic victory over its arch enemy the United States, which had tried to maintain an indefinite freeze on arms sales.

"As of today, all restrictions on the transfer of arms, related activities and financial services to and from the Islamic Republic of Iran... are all automatically terminated," the foreign ministry said in a statement.

The embargo on the sale of conventional arms to Iran was due to start expiring progressively from Sunday, October 18, under the terms of the UN resolution that blessed the 2015 nuclear deal between the Islamic republic and world powers.

"As of today, the Islamic Republic may procure any necessary arms and equipment from any source without any legal restrictions, and solely based on its defensive needs," the ministry added in the statement sent out on Twitter.

It insisted that under the terms of the deal, struck with the United States, China, Britain, France, Germany and Russia, "the lifting of arms restrictions and the travel ban were designed to be automatic with no other action required."

US President Donald Trump withdrew his country from the nuclear deal in 2018 and has unilaterally begun reimposing sanctions on Iran.

But Washington suffered a setback in August when it failed to win support from the United Nations Security Council to indefinitely extend the arms embargo.

- 'Momentous day' -

It was "a momentous day for the international community," the Iranian ministry said on Sunday, adding the world had stood with Tehran "in defiance of the US regime's efforts".

But it stressed that "unconventional arms, weapons of mass destruction and a buying spree of conventional arms have no place in Iran's defense doctrine".

Despite pulling out of the nuclear deal, the Trump administration insists it is still a "participant" and can therefore go ahead with reimposing sanctions.

Washington has said it has decided to unilaterally reinstate virtually all of the UN sanctions on Iran lifted under the accord.

But the US legal argument has been rejected by almost the entire UN Security Council, with European allies of the United States saying the priority is to salvage a peaceful solution to Iran's nuclear programme.

Iran urged the US to "abandon its destructive approach vis-a-vis Resolution 2231", adding that American attempts to "violate" the resolution had been "categorically rejected several times in the past three months by the Security Council".

The statement added that in the case of measures amounting to a "material breach of the resolution and the purposes" of the deal, Iran "reserves the right to take any necessary countermeasures to secure its national interests".

Moscow said in September that it was ready to boost its military cooperation with Tehran, while Beijing has also spoken of its willingness to sell arms to Iran after October 18.

Washington maintained it will seek to prevent Iran from purchasing Chinese tanks and Russian air defence systems.

Iranian Foreign Minister Mohammad Javad Zarif said in a tweet that the international community had "protected" the nuclear deal and Sunday marked the "normalization of Iran's defense cooperation with the world".